Infrared (IR) imaging sensors for displaying information are well known in the prior art. For some applications, IR sensors are used to distinguish an object from its background, and image data from several spectral bands is often useful for this application. The IR spectrum can be divided into mid-wave infrared (MWIR) and long-wave infrared (LWIR) bands, and prior art IR imaging systems typically are capable of displaying IR radiation in only one spectral band at any given time. For some applications, it is desirable to be able to simultaneously observe IR data in two LWIR bands and one MWIR band with the same sensor.
One way to observe multiple bands with the same sensor is to use a filter wheel. For this structure, a filter having sequential frame filters is provided. As the wheel is rotated, the different filter frames pass over the substrate, and different bands of IR radiation pass through and illuminate the substrate. Sequential frames can therefore collect image data in different bands. The disadvantage of this method, however, is that the IR image cannot be collected simultaneously in more than one band. Because of this, if there is a large relative motion between the imaged IR object and the sensor platform, this method causes a “blending” or “blurring” of the different IR bands. Thus, this method of providing a multi-color IR display is not particularly useful if either the imaged object or platform sensor, or both, is in motion.
Another way to collect IR information in a multi-band format is to provide an IR focal plane array (FPA) having multiple detector layers with multiple outputs at each pixel that correspond to the desired bands. U.S. Pat. No. 5,552,603, which issued to Burnham Stokes for an invention entitled “Bias And Readout For Multicolor Quantum Well Detectors”, discloses a device with this type of structure. For the invention as described by Stokes, three separate outputs are generated by assigning a detector layer to each desired band output and then electronically biasing each detector layer through a plurality of conductive layers that are interleaved among the three detector layers to be biased.
The result of the invention of Stokes, however, is an unnecessarily complex structure with three detection layers, each one optimized for a different band. Stated differently, and as mentioned above, the device recited by Stokes requires the deposition of extra detector and deposition layers and does not solve the ease of manufacturing question, in that it would also be very difficult and expensive to build. What is desired is an IR sensor that can simultaneously detect and display multiple “colors” (two from the LWIR and one from the MWIR band) from the same point in space without the complexity of a three detection layer structure. A focal plane array with two detection layers is much easier to fabricate than one with three layers.
In light of the above, it is an object of the present invention to provide an IR imaging device that can collect multi-band images. Yet another object of the present invention is to provide an IR imaging device that can simultaneously collect images in three distinct spectral bands. Another object of the present invention is to provide an IR imaging sensor that collects one band in the MWIR and two bands in the LWIR without loss of spatial resolution of the imaged object. It is another object of the present invention to use the diffraction effects of the LWIR in combination with two detection layers and an inhomogeneous (checker board or striped) filter to provide a three-color IR system. It is another object of the present invention to provide an IR imaging device that is relatively easy to manufacture in a cost-effective manner.